A number of different oligomerisation technologies are known to produce α-olefins. Some of these processes, including the Shell Higher Olefins Process and Ziegler-type technologies, have been summarized in WO 04/056479 A1. The same document also discloses that the prior art (e.g. WO 03/053891 and WO 02/04119) teaches that chromium based catalysts containing heteroaromatic ligands with both phosphorus and nitrogen heteroatoms, selectively catalyse the trimerisation of ethylene to 1-hexene.
Processes wherein transition metals and heteroaromatic ligands are combined to form catalysts for trimerisation, tetramerisation, oligomerisation and polymerisation of olefinic compounds have also been described in different patent applications such as WO 03/053890 A1; WO 03/053891; WO 04/056479 A1; WO 04/056477 A1; WO 04/056480 A1; WO 04/056478 A1; WO 05/123884 A2; WO 05/123633 A1 and U.S. Pat. No. 7,285,607.
The catalysts utilized in the abovementioned trimerisation, tetramerisation, oligomerisation or polymerisation processes all include one or more activators to activate the catalyst. Such an activator is a compound that generates an active catalyst when the activator is combined with the catalyst.
Suitable activators include organoaluminium compounds, organoboron compounds, organic salts, such as methyl lithium and methyl magnesium bromide, inorganic acids and salts, such as tetrafluoroboric acid etherate, silver tetrafluoroborate, sodium hexafluoroantimonate and the like.
A common catalyst activator used in combination with Cr based catalysts for oligomerisation of olefinic compounds is alkylaluminoxane, particularly methylaluminoxane (MAO). It is well known that MAO includes significant quantities of alkylaluminium in the form of trimethylaluminium (TMA), and in effect the catalyst activator is a combination of TMA and MAO. The MAO may also be replaced with modified MAO (MMAO).
Activators containing aluminium compounds are costly to the effect that it impacts significantly on process economics of olefin oligomerisation technologies that utilize this class of activators. For this reason, it is desirable to run commercial oligomerisation processes at low activator concentrations. However, in the case where an aluminium-containing compound was used as an activator for transition metal based oligomerisation catalysts, it was found that at conditions of low starting aluminium concentrations (e.g. <6 mmol/l), low reaction rates and high levels of unwanted solid formation (polyethylene (PE) and waxes) resulted when ethylene was oligomerised. This presented a major hurdle, since low final aluminium concentrations during catalysis is required and desirable for successful commercial operation.
The use of organoboron compounds as catalyst activators is known.
WO 07/088,329 relates to a transition metal catalyst system for the trimerisation and tetramerisation of olefins. The catalyst system comprises a transition metal compound, particularly chromium metal compounds, a diphosphine ligand and a catalyst activator. The specification mentions that the catalyst activator may be an organoaluminium compound, an organoboron compound or an inorganic acid and salt. However it contains no exemplification of the use of any of the organoboron compounds mentioned therein as activator. Such mentioned organoboron compounds as are mentioned in lines 18-23 on page 5 of that specification. The exemplifications of the processes for the trimerisation and tetramerisation of ethylene provided by WO 07/088,329 are all carried out in either chlorobenzene or toluene and no examples of processes conducted in aliphatic solvents are provided.
U.S. Pat. No. 5,919,983 teaches of a catalyst activator for use in the polymerization of α-olefins, using Ziegler-Natta and Metallocene polymerisation catalysts to form high molecular weight polymers. The activators taught are boron salts that respectively comprises a cation which is a Bronsted acid capable of donating a proton, and an inert, non-coordinating anion which includes a boron atom. The skilled person knows that Ziegler-Natta and Metallocene polymerisation technology belongs to a different art field and is fundamentally different to selective oligomerisation technologies.
In IPCOM000031729D, published on 7 Oct. 2004, boron-containing activators were used to activate selective oligomerisation catalyst systems in toluene or an aromatic solvent. When [Ph3C]+[B(C6F5)4]− and B(C6F5)3 was used to activate these catalysts, a low productivity catalyst was obtained with the highest productivities observed being around 15000 g/gCr.
It has now been found that the borate activators described herein leads to improved productivity of oligomerisation catalysts, when used in the presence of an aliphatic solvent.
The inventors of the present invention have accordingly found that using borate activators in the oligomerisation process described hereunder results in improved catalyst activation, increased catalyst efficiency and reduced solids formation, which improvements are herein collectively referred to as improved productivity of the activated catalyst.